Thin film magnetic recording media are composed of multiple layers disposed on a substrate, including one or more magnetic recording layers. Typically, the magnetic recording layer includes small magnetic grains that have an easy magnetization axis that is magnetically oriented longitudinally (i.e., in plane) with respect to the magnetic layer.
The areal density of such longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 gigabits per square inch (Gbit/in2) have been demonstrated. Scaling longitudinal recording media to higher areal densities requires smaller magnetic grains. However, as the grain size is reduced, thermal fluctuations can cause the magnetic domains to “flip”, resulting in a loss of magnetization over a period of time.
Perpendicular (i.e., vertical) magnetic recording media have been proposed as a way to increase areal densities beyond 100 Gbit/in2. Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is oriented substantially perpendicular to the magnetic recording layer. A perpendicular write-head, such as a monopole write-head or a shielded pole write-head, is utilized to magnetize the grains in the perpendicular recording layer.
The write-head for perpendicular recording media includes a write pole and a return pole, where the return pole is magnetically coupled to the write pole. An electrically conductive magnetizing coil surrounds the yoke of the write pole and is adapted to switch the polarity of the magnetic field applied to the write pole. During operation, the write-head flies above the magnetic recording medium by a distance referred to as the fly height, and an electrical current is passed through the coil to create a magnetic flux within the write pole. The magnetic flux passes from the write pole tip through the magnetic recording layer and into a magnetically soft underlayer (SUL) that is disposed beneath the magnetic recording layer. The SUL causes the magnetic flux to pass across to the return pole of the write-head. In addition, the SUL produces magnetic charge images during read operations, increasing the magnetic flux and the playback signal.
In a shielded pole write-head, a shield is disposed next to the write pole to increase the write field gradient. The shield, however, also leads to an additional flux path that consists of the write pole shield, the SUL, and the reader shields. Therefore, the write coil also induces flux through this additional path during writing, which can result in adjacent track erasure (ATE) that takes place underneath the write pole shield and/or underneath the reader shields. Given the large footprint of the shields, ATE can occur over a span as wide as 60 μm in the cross-track direction, a phenomena referred to as wide area ATE.
Since the SUL is part of the flux return path, the magnetic properties of the SUL play an important role in the ATE mechanism. For example, an anti-parallel coupled SUL (APS) can be used to suppress ATE and the reduced magnetic permeability (μ) in the presence of an APS has been implicated as one possible reason. Simulations have also suggested that perpendicular recording media having an SUL with a magnetic permeability of about 100 or less would have reduced ATE.
A competing property of the perpendicular magnetic recording medium is media overwrite (OW). Overwrite (−dB) is a measure of the over bias required to completely erase a lower frequency signal with a higher frequency signal. Data loss due to ATE could be mitigated by increasing the magnetic coercivity of the magnetic recording layer, however increased magnetic coercivity makes sufficient overwrite difficult to achieve.